Phenolic resole resin is used in the production of carbon-carbon composites intended for use in the manufacture of aircraft brake discs. Phenolic resins are used to produce molded carbon fiber preforms that will ultimately be carbonized and further densified and finally machined into the end product brake discs. Phenolic resins may also be used as reimpregnation resins after the initial carbonization of the preforms.
Phenolic resins are formed by reaction between phenol and formaldehyde. This is a condensation reaction, which releases water (H2O) as a byproduct. This condensation reaction proceeds by involving more and more phenol molecules. The resinous reaction product passes though a rubbery, thermoplastic state that is only partially soluble—this is called the B-stage. The C-stage occurs when the resin is cured and cross-linked to form a thermoset product. If the phenol/formaldehyde reaction is carried out in excess phenol with an acid catalyst, the reaction product is called a novolac resin. If the phenol/formaldehyde reaction is carried out in excess formaldehyde with a basic catalyst, the reaction product, at a low molecular weight stage, is called a resole resin.
In producing carbon-carbon composites, carbon fiber-reinforced phenolics are charred (pyrolyzed). This charring process results in a porous structure, because the phenolic ablates from solid to gas and does not go into a liquid phase. The resulting porous material is impregnated with pitch, phenolic resins, or directly with carbon by vapor deposition, and the resulting material is carefully charred again. This process is normally repeated several times until the voids in the composite are filled with carbon material. The process can take as long as 6 months, due to the need to prevent matrix damage.
Phenolic resole resins that contain a significant fraction of high molecular weight materials have been found to have improved resistance to cracking during processing. Phenolic resole resins with the appropriate fraction of high molecular weight materials have conventionally been made by extending the staging (or polymerization) step in the production of the phenolic resole resin. This approach increases the overall resin manufacturing process by several hours, and results in a very viscous resin component with a very high average molecular weight (undesirable characteristics) as well as the desired high molecular weight fraction.